Re: 4.14.9 with CONFIG_MCORE2 fails to boot

[Dave Hansen]

On 12/29/2017 10:46 AM, Alexander Tsoy wrote:
> Ð ÐÑ, 29/12/2017 Ð 09:32 -0800, Dave Hansen ÐÐÑÐÑ:
>> Does anyone have the results of build that they can share?ÂÂ(vmlinux,
>> vmlinuz/bzImage, System.map, .config).ÂÂThat, plus a corresponding
>> serial log with an oops would be helpful.
> 
> Here you are:
> https://www.dropbox.com/s/yesupqgig3uxf73/linux-4.15-rc5%2B.tar.xz?dl=0

Alexander, thanks a bunch for the quick turnaround on this.  It is much
appreciated!

With your binary, I can reproduce this in a KVM guest.  Seems we manage
to get to paranoid_entry with a kernel GS value, but the user page
tables in place.  We don't smash the #DF stack because we reset the
stack at each new #DF.  I think the loop that we get stuck in goes
something like this:

1. Hardware does #DF, calls double_fault
2. call paranoid_entry
3. check MSR for GSBASE, see it has kernel value, skip SWAPGS and
   switch to kernel page tables
4. touch stack, try to #PF, but can't touch stack, so #DF and goto 1

The real question is where we double-faulted from in the first place
with a kernel GSBASE and user CR3.  I think I just need to disable KASLR
and do a little work in gdb to look at the stack on the first
double-fault, but we'll see.